This invention relates to methods of chemically removing coatings from surfaces of components, such as components exposed to the hot gas path of gas turbines and other turbomachinery. More particularly, this invention is directed to a method of masking regions of a component before chemically stripping a coating from the component with a HxAF6 acid-based stripping solution, where A is silicon, germanium, titanium, zirconium, aluminum or gallium, and x has a value of one to six.
The operating environment within a gas turbine is both thermally and chemically hostile. Significant advances in high temperature strength, creep resistance, and fatigue resistance have been achieved through the formulation of iron, nickel and cobalt-based superalloys. However, components in the hot gas path of a gas turbine, such as the buckets, nozzles, combustors, and transition pieces of an industrial gas turbine, are susceptible to oxidation and hot corrosion attack. Consequently, these components are often protected by an environmental coating alone or in combination with a ceramic thermal barrier coating (TBC), which in the latter case the environmental coating is termed a bond coat for the TBC. Components protected by an environmental coating or TBC system exhibit greater durability as well as afford the opportunity to improve efficiency by increasing the operating temperature of a gas turbine.
Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a stable, adherent, and slow-growing aluminum oxide (alumina) layer (or scale) at elevated temperatures. Notable examples include diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide and platinum-modified nickel aluminides (PtAl), and overlay coatings such as MCrAlX alloys (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or a rare earth or reactive element) or aluminide intermetallics (e.g., beta-phase and gamma-phase nickel aluminides). The alumina scale grown by these coatings protects the coatings and their underlying substrates from oxidation and hot corrosion and promotes chemical bonding of a TBC (if present). Diffusion aluminide coatings are formed by diffusion processes such as pack cementation, above-pack, and chemical vapor deposition techniques, and are characterized by an outermost additive layer containing an environmentally-resistant intermetallic represented by MAl, where M is iron, nickel, or cobalt, depending on the substrate material, and a diffusion zone beneath the additive layer and comprising various intermetallic and metastable phases that form during the coating reaction. Diffusion coatings are particularly useful for providing environmental protection to components with internal cooling passages, such as turbine buckets, because of their ability to provide environmental protection without significantly reducing the cross-sections of the passages due to the minimal thickness of the additive layer. In contrast, overlay coatings are predominantly an additive layer with limited diffusion zones as a result of the methods by which they are deposited, which include thermal spraying and physical vapor deposition (PVD) processes.
Though significant advances have been made with environmental coating, bond coat, and TBC materials and processes for forming such coatings, there is the inevitable requirement to repair or remove these coatings under certain circumstances. For example, removal may be necessitated by erosion or thermal degradation of an environmental coating or bond coat, refurbishment of the component on which the coating was deposited, or an in-process repair of the coating. Current state-of-the-art repair methods for removing ceramic and metallic coatings of the type used in TBC systems include grit blasting and treatments with an acidic stripping solution. The latter typically rely on lengthy exposures to the stripping solution, often at elevated temperatures, that can cause significant attack of the underlying metallic substrate, such as alloy depletion and intergranular or interdendritic attack. Furthermore, as in the case of the internal cooling passages of a turbine bucket, removal of an environmental coating is often undesirable and unnecessary. To selectively remove coatings from only those surface regions requiring refurbishment, masking materials are applied to those surfaces requiring protection from the stripping solution. As an example, to protect the interior passages of air-cooled turbine engine components, low-melting waxes and thermosetting resins such as plastisols have been injected into the cooling passages. An advantage of plastisol is, after curing at high temperature, its ability to withstand elevated temperatures used with acidic stripping solutions. After stripping the coatings from the unmasked surface regions, the masking material must be removed. In the case of low-melting waxes, removal of the masking material can be performed in a low temperature furnace. In contrast, plastisol requires a high temperature burn out that produces hazardous gases which must be scrubbed from the exhaust.
An improved acidic stripping solution disclosed in commonly-assigned U.S. Pat. No. 6,833,328 to Kool et al. is an aqueous solution containing an acid of the formula HxAF6 and/or precursors thereof, where A is silicon, germanium, titanium, zirconium, aluminum, or gallium, and x has a value of one to six. The stripping solution taught by Kool et al. may further contain one or more additional acids, such as nitric acid, a phosphorous-containing compound such as phosphoric acid, a mineral acid such as hydrochloric acid, etc. As taught in commonly-assigned U.S. Pat. Nos. 6,599,416, 6,758,914, 6,793,738, 6,863,738, and 6,953,533 and U.S. Patent Application Publication Nos. 2004/0074873 and 2004/0169013, the acidic solution of Kool et al. is effective to remove a variety of coating compositions, including diffusion aluminides, diffusion chromides, MCrAlX overlay coatings, and the oxide layers that grow on these coatings, without significantly attacking the substrate beneath these coatings. Another advantage of the Kool et al. solution is that, from an environmental standpoint, the HxAF6 acid is relatively benign in comparison to mineral acid-based compositions. Nonetheless, there are circumstances in which surfaces of a component being stripped with this solution are preferably protected. A notable example is the internal cooling passages of gas turbine components whose internal surfaces are protected with an environmental coating, particularly diffusion aluminide coatings, towards which the HxAF6 acid of Kool et al. is aggressive. However, low melting waxes cannot withstand treatment temperatures (typically about 80° C.) preferred for the HxAF6 acid stripping solution, and thermosetting resins such as plastisols are undesirable because of their requirement for a high temperature burn producing hazardous gases.
From the above, it would be desirable to provide a process by which a HxAF6-based acidic stripping solution can be prevented from attacking certain surface regions that are prone to attack from the solution.